Hi Jens,
There is no built in way of turning off the tooltips in Safari as far as I know.
You can though turn them off and on by running a terminal command
if you feel comfortable doing so.
To do this:
To turn off the tooltips, users should open the Terminal application and enter the following command:
defaults write com.apple.Safari WebKitShowsURLsInToolTips 0
To reverse this command, enter it again with a "1" at the end instead of the "0"
This is from a MacFixit fixit found at:
http://reviews.cnet.com/8301-13727_7-10329800-263.html
I believe that third party utilities such as Cocktail or TinkerTool also have the capability of turning off tooltips which you might prefer if not comfortable working with terminal app.
hope that helps some,
LS 🙂